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The production of ethanol from
starch using most of the common fermentation microbes has been low.
However, with the adoption of gene manipulation
recombinant microbes have been engineered that seem capable of
directly converting starch into alcohol and carbon dioxide. Such
engineering, however, invariably adds to the cost of adopting the
technology towards to production of ethanol from starch. A process
that eliminates that approach should be cost effective at least
relatively. The concept is to have the starch get converted into
sugar and the sugar into ethanol with both fermentation reactions
occurring simultaneously.
This analysis centers on the
development of a dual fermentation packed bed batch reactor to
accomplished the starch to ethanol conversion process. Conceptually, a multi packed bed biofilm batch reactor
using mixed microbe biofilms would accomplish the objective. A configuration of a concept
Multi Packed-Bed Biofilm Batch Dual-Fermentation Reactor to adopted for the
purposes of the analysis may have the same description a single
packed bed
biofilm batch ethanol reactor, but for the packed bed. This
choice invariably avails this reactor with all the intrinsic
advantages of the template single packed bed reactor.
However, because of the
complex need of this bioreactor, design of the
Fermentation
Mash Feeder equipment integrated with the reactor must
also take into consideration the diverse need of the many biofilms
of the packed bed. The specific configuration of the equipment which
is application-specific being dependent on the
reactants and
catalysts that must be added to the substrate to be termed the
Feed-Mash
necessary to also support the anabolic reactions for cell
maintenance, must now also take into consideration any and all
conflicting demands of the biofilms .
The substrate mixture is
expected to consists of starch from any source and the
cell-functions
enhancement substances, required to support the metabolic
reactions.
The Dual-Biofilm Multi-Packed Bed
The multi packed bed for the
bioreactor is designed to have the dimensions of half the diameter
of the reactor, and of height less than the height of the lateral
cylinders. The overall bed height is made variable so as to be
changed on the basis of the results of the analysis. The beds are
pre-packed in cylindrical beds that are stacked one on top of the
other with a small gap between the stacking. The upper bed is a pack
of beads on which have been formed biofilm of an
ethanol fermentation bacteria, Zymomonas mobilis. The lower bed
is a pack of
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beads which forms biofilm of entrapped
starch fermentation yeast, that carries the α-amylase gene, such
as Saccharomycopsis fibuligera. Because
the reaction mixture will start off as starch and is expected to
slowly become a mixture of starch and sugar, the mixture should be
non-mechanically convected within the reactor to support continuous
access of the substrates to the
biofilms.
Then enabling the non-mechanical convection
of the reaction mixture, the
bubbling flow
conditions of the packed bed reactor shall be taken advantage
of; and for that reason, a channel is placed some distance above the
upper bed, to concentrate the bubbling flow into a narrower
cylindrical flow path, with the object of inducing vigorous
upwelling flow and consequentially downward circulatory flow.
A critical ratio of bed porosity
ratio is maintained so as to allow fast flow through the lower bed
while maintaining a much slower flow through the upper bed. This
slow flow in the upper bed supports the requirement of a near
quiescent
reaction mixture so as to maintain,
the
stagnation fluid condition preferred to allow spontaneous
self-immobilization in the upper bed of the new cell bacteria produce in course of
the reaction.
Microbes Immobilization
Techniques
The biofilm formation
processes follow standard practices. The Zymomonas mobilis biofilm
is formed by allowing the bacteria to self- organize into the sessile
state over the beads. The yeast biofilm is formed by
entrapment immobilization such as could be used to form biofilms.
Thought-Analysis of the
Reactor Dynamics
The reaction
substrate mixture of a volume such as will completely cover up the
upper bed packed bed is pumped into the reactor from the feed tank. and
the charge is pumped at a rate that is fast enough to
overwhelm the initial reaction during the charging of the feed.
The reaction now starts from
within the lower bed, with the microbes converting the starch into
sugar. Within the reaction mixture in the gap between the beds, the
starch substrates diffuses downwards into the lower bed under the
chemical potential gradient created by the reaction in the lower
bed. In the counter direction, the sugar produced by the reaction
diffuses upward into the fluid between the the beds again due to the
chemical potential gradient caused by the reaction.
As a result of the
reaction, the microbes of this lower bed grow inside the entrapment
spaces causing a swelling of the entrapment beads. This enlargement
of the beads causes further increases of the porosity of the bed
setting the conditions for |
even faster upward flow of
the reaction-substrate mixture, as well as increasing the height of
the bed. As the sugar diffusing upwards gets into the upper bed, the
microbes of that bed then begins to ferment the sugar into ethanol
and dissolved carbon dioxide. The ethanol produced by the
fermentation reaction diffuses upward into the fluid above the upper
bed again due to the chemical potential gradient caused by the
reaction and the upwards drift caused by the upwards diffusing
sugar. Similarly, the dissolved carbon dioxide molecules, also
created from the reaction, also travels upwards into the bulk of the
fluid above the upper bed.
The bacteria of the upper bed
also grows in a manner described by the Monod
equation; and the new cells of bacteria also, as expected,
self-immobilizes onto the carrier beads
and remain in the sessile state but carries on the fermentation as
the predecessor microbes.
At a certain thermodynamic
condition, the dissolved carbon dioxide diffusing into the fluid
above the upper bed, begins to undergo homogeneous nucleation and
spontaneously forms bubbles and creates a two phase system, in which
the carbon dioxide bubbles are traveling upwards and out of the
reaction mixture into the space above the reaction mixture. The
two-phase flow is, of course, channeled into a narrower flow path
and therefore creates more intense flow conditions resulting in more
vigorous convective flow
of the fluid upwards and invariably causes recirculation of
the fluid forcing the flow of the fluid downwards on the sides of
the beds or support frames and upwards from the bottom of the reactor
through the lower bed and then the upper bed.
The prevailing
spontaneous fluid flow now provides the needed convective flow that
forces more starch to the lower bed microbes, and sugar to the upper
bed microbes.
Developing a Computational
Analysis
Obviously, this spontaneous
change of the fluid dynamic characteristics must be
captured in the model equations of the
computational analysis as to be able to predict the precise time
when the fluid dynamics changes. The model must also accurately
predict the height of the dual packed bed
that effects the gas bubble formation.
Significant Advantage
The reaction should be effective
enough as the conversion of the sugar into ethanol removes a
product-inhibiting effect of the sugar on the Saccharomycopsis
fibuligera, while eliminating the need for the gene manipulation |
